This invention relates to the field of three-dimensional digital image capture, and more particularly, three-dimensional image capture of people using digital stereo photogrammetry. Digital stereo photogrammetry is a technique for the recovery of the three-dimensional attributes of an object by the use of pairs of digital photographs, typically, but not necessarily, taken by a pair of cameras. Provided that the positions, orientations and focal properties of the cameras used to take the images are known, it is possible for a computer to estimate the distance between either of the cameras and an object appearing in the images taken by both of them.
The computer does this by determining which group of contiguous pixels in an image taken with the second camera match up with a pre-specified group of contiguous pixels in an image taken with the first camera. From this, a parallax can be computed, and from that, using simple geometry, the distance to the object, light from which produced these pixels, can be derived.
The ability of a computer to correctly match corresponding areas of the images from the two cameras is dependent upon there existing, within each area, significant variations in image intensity. If an area of an object is visually xe2x80x98flatxe2x80x99, that is to say, of uniform visual intensity, then there will be potential for ambiguity in determining the position of matching points on the two images. Such visual flatness normally arises because an object or subject being imaged is lit by a uniform source of illumination and has areas on their surface or on their skin which differ little in albedo.
In principle the uniformity of image intensity can be obviated in two ways. One can change the albedo of the surface, for example by painting patterns with make-up on a person""s face, or alternatively, one can vary the intensity of illumination across the surfaces being imaged.
For certain applications it is convenient to capture both the three-dimensional shape of an object and its associated visual texture, for instance when capturing both the appearance and three-dimensional shape of an actor""s face. This makes the use of visually disruptive makeup unattractive. The alternative of illuminating the subject with textured light is used in known systems such as the Turing C3D system.
The state of the art technique for illuminating a subject with textured light involves the use of a slide projector which is set to produce a focused image of a random dot pattern on the face of the subject. The process involves taking an initial pair of images using textured light, a subsequent third image is then taken with the slide projector illuminating the subject through a uniform gray slide. The third image, having been taken using uniform light intensity can be used to reconstruct the subject""s skin tone in the ultimate computerised three-dimensional model.
Whilst this approach produces reasonably good three-dimensional models it does suffer from a number of practical disadvantages. One of these is that the subject has to stare into a bright light coming from the projector. To allow for sufficient depth of field the aperture of the projector must of necessity be small. Intense illumination subtending a small angle of the field of view of the eye has recently been brought under various international health and safety regulations which render the legality of such a system questionable. Whether safe or not the experience of staring into an intense light is unpleasant for the subject and does not facilitate the capture of natural and relaxed expressions.
Since the duration of the exposure is not well controlled, there is a danger that the infrared loading on the retina from the high intensity lamp in the slide projector may exceed safe limits. A second drawback is the imperfect registration between the textured and white light images consequent upon slight movements by the subject during the second or so that it takes to switch between textured and white slides. A third disadvantage relates to the bulk and power consumption of slide projectors. These are typically heavy devices requiring main power for their operation. This precludes their being mounted on photographic tripods, or being incorporated into a portable system.
This invention, which is defined in the appended claims, seeks to obviate the above disadvantages of the state of the art. It consists of a high depth of field flash projector, preferably battery powered, This has the advantages over a standard slide projector for three-dimensional image capture of people using digital stereo photogrammetry that the energy delivered in a flash can be precisely calibrated and it is possible to ensure that this falls below a level that might pose a danger to the retina of the subject.
The intensity of light during the instant of the camera""s exposure can be far greater than the intensity of a practical continuous light source even though the total energy delivered to the subject is substantially less than from a continuous source. This facilitates smaller apertures providing greater depth of field and also allows the projection optics to cover a wider angle than is practical with a continuous source. This means that the overall volume required for a three-dimensional capture system and subject can be substantially reduced.
Because a high level of illumination only has to be maintained for a few milliseconds, power to the projector can be derived from a battery making the system portable. The flash projector is light-weight and can be mounted on photographic tripods.